The present invention relates to a method of compressing halftone image data applicable to an electronic publishing system, fasimile transmission system, telephone with a display, videotex, image filing system, etc.
A problem with an electronic publishing system, for example, is that a photograph or similar image having gradations, i.e., a natural image containing halftone is far greater than characters or two-level images with respect to the amount of data and is therefore not feasible for a filing or similar purpose. This problem may be eliminated by dividing an image into lines and compressing gradients along the lines, as disclosed in Japanese Laid-Open Patent Publication No. 62-135073. This kind of approach, however, does not give any consideration to the correlation in the direction perpendicular to the lines. Considering a vertical stripe pattern, for example, the compression efficiency attainable with the disclosed approach is poor, while decreasing the amount of codes aggrevates the image quality.